Cheesy Chicken Alfredo Soup

In a large pot over medium-high heat, melt butter. Cook chicken until cooked through and golden, 6 minutes per side, then season with salt and pepper and set aside. Once cool, dice. Add onion to pot and cook until soft and golden, 6 minutes, then add garlic and cook until fragrant, 1 minute. Add flour and whisk until golden, 2 to 3 minutes, then add chicken broth and half and half and simmer until thickened, 15 minutes. Add tortellini to pot and cook until al dente, 10 minutes. Add diced chicken and season with crushed red pepper flakes, salt, and pepper. Ladle into bowls and garnish with basil.